Expression in Host Cells
Advances in vaccine production techniques have made it possible to synthesize polypeptides corresponding to the HBs antigen in bacteria, yeast and mammalian cells. Transcription of eukaryotic genes in bacteria and yeast, however, adversely affects the efficaciousness of these polypeptides as antigens due to several drawbacks concerning the glycosilation and secretion of the polypeptides and composition of the particle formed therefrom.
For example, in the case of the Hepatitis B virus, the polypeptide antigens produced in vivo are heavily glycosilated (Gerlich, 1984: J. Virol.: 52 (2), 396). In prokaryotes, glycosilation is not an essential process so that polypeptides produced by genetically engineered bacteria are either not glycosilated or are incompletely glycosilated. In either case, polypeptides corresponding to HBsAg, when expressed in bacteria, do not raise antibodies which will see HBsAg sufficiently well for an effective vaccine. Although yeast as a eukaryotic host is capable of more complete glycosilation, polypeptides corresponding to HbsAg expressed in yeast share the same deficiency as in the case of bacterial expression. (Murray et al., 1979: Nature, 282, 575; Valenzuela et al., 1982: Nature, 298, 347; Miyanohara et al., 1983: PNAS, 80, 1).
As a further example, in bacteria the eukaryotic structural gene of the HBsAg is in most cases not efficiently transcribed. Furthermore the structure and function of the eukaryotic HBsAg gene product may be dependent on the additional post-translational processes of the linkage of disulfide bonds which can not be accomplished by the bacterial host.
Still further, the expressed polypeptide is rarely secreted from the bacterial host cells. They must be lysed to harvest the expressed polypeptide. During the purification process bacterial wall components may contaminate the polypeptide and cause serious allergic reactions or lead to anaphylactic shock in patients.
Finally, eukaryotic promoters usually do not work in bacteria and must be substituted by a bacterial promoter which can result in modification of the polypeptide expressed. (Offensperger et al., 1985: PNAS, 82, 7540; Valenzuela et al., 1980: ICN-UCLA Symp, Mol. Cell. Biol., 18, 57).